Grey water heat recovery system

ABSTRACT

A grey water to clean water heat exchanger and methods of use. A heat exchanger having a copper coil carrying clean water inside a grey water discharge pipe is described. The heat exchanger can use counterflow geometry. The heat exchanger has a controller that directs grey water through a bypass pipe whenever the grey water has too low a temperature to provide useful thermal energy. When the grey water is hot enough, the controller directs it through the heat exchanger to heat the clean water. A filter is provided to eliminate fouling of the interior of the heat exchanger. The controller provides an indication when the filter needs to be cleaned or replaced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of co-pending U.S.provisional patent application Ser. No. 61/266,275, filed Dec. 3, 2009,which application is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to heat recovery systems in general andparticularly to a system that recovers heat from waste water.

BACKGROUND OF THE INVENTION

There is a growing need for a simple system that recovers energy fromhot commercial wastewater. The equivalent of 235 billion kWh worth ofhot wastewater is discarded annually in the United States according tothe U.S. Department of Energy. A significant amount of the energy fromthis wasted hot water could be recovered. Desirable features of such asystem would include easy installation, minimal impact on existingplumbing and low maintenance. Over time a wastewater recovery systemwill be able to pay for itself through savings earned from energyconservation. Energy prices have increased exponentially over the pastfew years.

Each day thousands of gallons of hot wastewater (also known as greywater) go down the drain, taking valuable heat along with it. A largeamount of this lost energy could be used to preheat incoming cold water.There are several devices currently on the market that can take thewastewater from one household shower and transfer its energy to the coldwater going to the shower. Heat energy is also lost in other waste watersystems used in residential and commercial environments. This loss ofenergy is especially prevalent in businesses or commercial locationssuch as residence halls, barracks, gym locker rooms and laundromatswhere large amounts of hot water are being used continuously throughoutthe day.

Heat exchangers are common devices used to transfer heat from one mediumto another. This transfer of heat can be used to recover potentiallywasted energy from hot grey water.

As energy conservation becomes an ever pressing issue on the minds ofpeople, businesses and governments around the world, new methods ofconserving energy are continuously under development. The rising cost ofnatural gas, oil and electricity helps to solidify the importance ofenergy conservation in modern society. One area where energy is wastedon a regular basis is the hot water being sent down drain aftershowering, washing hands, and doing the laundry. This wasted water isknown as grey water and is fast becoming a new area of research andchange in the initiative to “go green”. Today there are four devices onthe market that are available to assist in capturing this wasted energy.None of these devices are rated to capture more than fifty percent ofthe energy within the grey water going down the drain or to handle thecapacity of more than two full bathrooms. The four systems are describedin the following articles: “How the Power Pipe Works,” RenewABILITYEnergy Inc., 2006; “Lo-Copper GFX,” Heat-Xchangers and Water HeaterBoosters, WaterFilm Energy Inc., 2002; “ReTherm Heat RecoveryTechnology,” Discover Heat Recycling, ReTherm Energy Systems Inc.,2004-2008; and “Drain Water Heal Recovery,” WaterCycles, 2007.

Other examples of heat exchangers for such systems that are described inthe prior art include the following United States patents.

U.S. Pat. No. 1,703,655, “Heat Exchanger for Waste Process Water,”teaches a very large and advanced system; the system utilizes storagetanks for the incoming heated wastewater which is of concern due tosanitary conditions. The device uses multiple pipe passes to increasethe surface area for heat transfer in a tube bundle design.

U.S. Pat. No. 4,256,170, “Heat Exchanger,” teaches a liquid-to-liquidheat exchanger that transfers heat from hot wastewater to the coldsupply water. This is a pipe in a pipe design that is composed of aninner pipe and outer shell in which the outer shell is sub-divided intoa serpentine flow path for the cold water. The inner pipe is sized sothat flow of the hot water does not need to completely fill the pipe,this design allows for less than full pipe Howl.

U.S. Pat. No. 4,291,423, “Heat Reclamation for Shower Baths,” teaches asystem intended for a bath or shower unit, the design utilizes hot drainwater that is leaving the shower and uses it to heat the incoming coldwater. The piping is designed to be placed underneath a tub or shower.The coils are located in a basin at the bottom of the tub which allowsthe lines to stay submerged under the wastewater while the shower is inuse.

U.S. Pat. No. 4,304,292, “Shower,” teaches a system designed to beinstalled in a tub or shower unit and includes two different possibleconfigurations. Both configurations use the warm shower wastewater toheat the incoming cold water supply. The first configuration is a normalplumbing drain trap made of copper with a copper helical pipe wrappedaround the outside. The second configuration is a standard plumbingdrain tap with a copper helical pipe placed on the inside of the draintrap.

U.S. Pat. No. 4,341,263, “Waste Water Heat Recovery Apparatus,” teachesa wastewater heat exchanger that is designed to capture the grey waterfrom multiple household or commercial drains and then pass it throughthe exchanger. This system uses counter flowing pipes and a liquid heatexchanger medium to better accommodate the transfer of heat between hotand cold flows.

U.S. Pat. No. 4,372,372, “Shower bath Economizer,” teaches a helicalpipe in a pipe heat exchanger design. This design is intended to conductheat from spent shower wastewater and transfer it to the incomingwastewater. The heat exchanger has at least two concentric helical coilsthrough which the incoming cold water flows and over which the spentwastewater flows. A central core directs the spent water between thehelical coils for good heat transfer.

U.S. Pat. No. 4,542,546, “Heat Recuperator Adapted to a Shower-Cabin,”teaches a system that utilizes a hot water basin placed under the showerwhich stores the hot wastewater. This warm stored water is then used topre-heat incoming cold water. The cold water pipe passes through theshower tank on the way to the shower head, heating as the flow passesthrough the tank.

U.S. Pat. No. 4,619,311, “Equal Volume, Contra Flow Heat Exchanger,”teaches a system that utilizes the hot wastewater leaving a householdshower and sends it through a heat exchanger which passes the incomingcold water across it. With this design the water travels extremely fastthrough the system due to the fact it travels straight down the pipewith no obstructions, this would require the pipe to be extremely longto recover a beneficial amount of heat. This is due to the fact theamount of time the water has to fully transmit the heat to the incomingcold water is significantly decreased in a pipe in pipe design that is ashort length.

U.S. Pat. No. 4,821,793, “Tub and Shower Floor Heat Exchanger,” teachesa heat exchanger that is located in the floor area of a bathtub orshower. The device extracts heat from spent shower or bath water anduses it to heat the incoming cold water to reduce the amount ofpreheated boiler water needed to be sent to the shower.

U.S. Pat. No. 5,143,149, “Wastewater Heat Recovery Apparatus,” teaches adesign that includes a valve that controls the hot and cold watersentrance and exit from the heat exchanger with pipe in pipe heatexchanger configuration. The cold water enters the pipe flowing in theopposite direction of the heated wastewater causing a counter-flowcondition.

U.S. Pat. No. 5,301,745, “Installation for Heat Recovery,” teaches afacility producing warm drain water and consists of a distributor tank,many pipelines, and a heat exchanger. The distributor tank reads thetemperature which is then conveyed to a mixing valve which adds warmwater or cold water back into the storage tank. This allows the storagetank to maintain a set temperature.

U.S. Pat. No. 5,740,857, “Heat Recovery and Storage System,” describes awater system that recovers heat from hot wastewater and transfer it to acooler water supply. The heated water is then stored in a tank untilneeded for use. The patent teaches a high thermal conductivity pipeenclosed by a low thermal conductivity pipe. These pipes consist of aninlet on one end and an outlet coupling on the other. There are alsoendplates that are adapted to accommodate other household pipingallowing the device to take water from more devices then just a shower.

U.S. Pat. No. 5,791,401, “Heat Recovery for Showers,” teaches a systemthat is intended to be placed right in the wall of the shower. Thisdesign also uses the hot grey water to heat the incoming cold watergoing to the shower head. This is a copper pipe on a copper drain pipedesign which has been discussed previously, the increased spacing inthis design decreases the contact area the cold water flow has toexchange heat.

U.S. Pat. No. 7,322,404, “Helical Coil-on-Tube Heat Exchanger,” teachesa device that consists of copper tubing wrapped around the outside of acopper drain pipe. This design is used in all of the competing devicescurrently on the market. This patent uses multiple parallel helicaltubes to limit liquid pressure losses while providing similarperformance to other coil and tube designs. Two or more copper tubes arewrapped around a pipe in a helical shape allowing the heat to betransferred in a counter-flow fashion. This system contains a header ormanifold to allow more than one helical pipe to be connected.

There is a need for a commercial device that can accommodate the flow ofmultiple showers or other large volumes of grey water.

SUMMARY OF THE INVENTION

According to one aspect, the invention features a grey water to cleanwater heat exchanger. The grey water to clean water heat exchangercomprises a discharge pipe having an exterior surface, an interiordiameter, a grey water inlet and a grey water outlet, the discharge pipeconfigured to convey grey water from a location of use to a dischargelocation; a helical coil having a clean water inlet and a clean wateroutlet, the helical coil situated within the interior diameter of thedischarge pipe with the clean water inlet and the clean water outletconfigured to be accessible from locations outside the exterior surfaceof the discharge pipe, the helical coil configured to carry clean waterfrom a water source to location of use of the clean water, the dischargepipe and the helical coil configured to allow heat energy to flow fromthe grey water to the clean water; a filter configured to be removablypositioned within the grey water flow, the filter configured to filtermaterial from the grey water; a bypass valve and a bypass pipeconfigured to allow grey water to controllably bypass the discharge pipeand the helical coil and to reach the discharge location; and acontroller configured to activate and deactivate the bypass valve inresponse to a non-volatile control signal.

In one embodiment, the discharge pipe and the helical coil areconfigured to have the grey water and the clean water flowing incounterflow to each other.

In another embodiment, the controller is a thermal switch.

In yet another embodiment, the thermal switch is a bimetallic element.In still another embodiment, the controller is an electronicmicroprocessor-based controller configured to operate under instructionsprovided on a machine-readable medium.

In a further embodiment, the controller further comprises a thermalsensing element.

In yet a further embodiment, the thermal sensing element is athermocouple.

In an additional embodiment, the controller further comprises a flowsensing element.

In one more embodiment, the helical coil comprises copper.

In still a further embodiment, the filter comprises a nylon filterelement.

According to another aspect, the invention relates to a thermal energyrecovery method. The thermal energy recovery method comprises the stepsof: providing an grey water to clean water heat exchanger, comprising agrey water discharge pipe configured to carry grey water and a helicalcoil situated within the discharge pipe, the helical coil configured tocarry clean water, a removable filter configured to filter material fromthe grey water, a bypass valve and a bypass pipe configured to allowgrey water to controllably bypass the discharge pipe, and a controllerconfigured to activate and deactivate the bypass valve in response to anon-volatile control signal; providing a source of grey water and asource of clean water; setting the bypass valve to convey grey waterthrough the bypass pipe; sensing whether a flow of grey water having atemperature above a predetermined value is present in the bypass pipe;in response to sensing that a flow of grey water having a temperatureabove a predetermined value is present in the bypass pipe, causing thecontroller to operate the bypass valve to flow the grey water throughthe discharge pipe; and recovering thermal energy from the grey waterand heating clean water in the helical coil with the recovered energy.

In one embodiment, the controller controls the bypass valve to causegrey water to flow through the bypass pipe when the filter becomesclogged.

In another embodiment, the controller indicates that the filter requirescleaning or replacement when the filter becomes clogged.

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent from the following descriptionand from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below, and the claims. The drawingsare not necessarily to scale, emphasis instead generally being placedupon illustrating the principles of the invention. In the drawings, likenumerals are used to indicate like parts throughout the various views.

FIG. 1A is a diagram in axial view of the exterior pipe and interiorpipe in a pipe heat exchanger.

FIG. 1B is a diagram in cross-sectional view of the exterior pipe andinterior pipe in a pipe heat exchanger, in which counter flow isemployed.

FIG. 2 is a perspective view of a helical coil in a pipe heat exchanger.

FIG. 3 is a diagram in axial view of a helical coil in a pipe cross flowheat exchanger.

FIG. 4 is a diagram in vertical cross-sectional view of the helical coilin a pipe counter flow heat exchanger.

FIG. 5 is a diagram of a copper helical coil used in the heat exchangerof FIG. 3 and FIG. 4.

FIG. 6 illustrates a heat exchanger location under a shower room.

FIG. 7 is a schematic diagram illustrating a heat flux representation ofone design.

FIG. 8 is a graph showing the outlet temperatures of water flows in aheat exchanger as a function of length.

FIG. 9 is a graph showing the prototype cost (straight line) and yearlyenergy cost savings (curved line) as a function of heat exchanger lengthin feet.

FIG. 10 is a graph showing the net saving as a function of heatexchanger length in feet for the example illustrated in FIG. 9.

FIG. 11 is an illustration of a test set-up for a heat exchangerconstructed according to the principles of the invention.

FIG. 12 is an illustration of a restricted flow test setup for a heatexchanger constructed according to the principles of the invention.

FIG. 13 is a graph showing the results of restricted flow test 1.

FIG. 14 is a graph showing the results of restricted flow test 2.

FIG. 15 is a graph showing the results of restricted flow test 3.

FIG. 16 is an illustration of an unrestricted flow test setup for a heatexchanger constructed according to the principles of the invention.

FIG. 17 is a graph showing the results of unrestricted flow test 1.

FIG. 18 is a graph showing the results of unrestricted flow test 2.

FIG. 19 is a graph showing the results of unrestricted flow test 3.

FIG. 20 is a perspective diagram of one embodiment of a helical coilcounterflow heat exchanger in assembled form.

FIG. 21 is a perspective diagram of one embodiment of a filter modulebuilt and used according to principles of the invention.

FIG. 22 is an exploded diagram of one embodiment of a helical coilcounterflow heat exchanger that illustrates the parts of the invention.

FIG. 23 illustrates one embodiment of the system, which includes atemperature valve 2301 allows cold water to bypass the heat exchanger.This valve also allows for backed up water to go through the bypass pipe2303. The filter 2302 is provided to prevent dirt in the grey water fromclogging the interior of the heat exchanger.

FIG. 24 illustrates another embodiment of the system, which includes aclosed off pipe through the middle of the heat exchanger to distributethe flow and ensure water flow over the helical pipe.

FIG. 25A illustrates a housing for a filter configuration.

FIG. 25B illustrates a plug that is inserted into the housing for thefilter configuration. The plug and the housing are threaded and can bethreadedly connected.

FIG. 26 illustrates a filter configuration having a baffle built intothe filter mechanism and can be used any of the embodiments.

FIG. 27A illustrates a filter handle that mates with a filter plugillustrated in FIG. 27B. The handle and plug fit together with a tightfit holding them in place. There is a built in baffle and there aredistributor tubes within the filtering mechanism.

FIG. 28A illustrates a filter handle and plug that mates with a filterplug illustrated in FIG. 28B. The handle and plug fit together with atight fit holding them in place. This embodiment comprises the firstfiltering mechanism described herein.

FIG. 29 illustrates a manufacturing improvement to allow this product tobe fully assembled without welding. Welding is a distinct problem whenfitting the entire assembly together. By having thread ends with anoutside piece being the male or female threading, depending on the coilsmale or female threading, the two mate allowing for a water tight fit.The other end of the outside pipe can have a standard joint.

DETAILED DESCRIPTION

We describe an effective wastewater heat recovery system through the useof a simple heat exchanger that can accommodate multiple grey waterproducing devices. An efficient heat exchanger using a copper helicalpipe design has been shown to provide heat recovery of approximately40.5° F. and a cost savings of approximately $1500 per year. Through theuse of this design businesses could significantly reduce overheadoperating costs.

A design that would meet the needs of commercial businesses could beinstalled in a building's mechanical room or basement. The device wouldfit within an 8 inch by 8 inch by 8 foot box, these dimensions allow thedevice to be retrofitted easily into a typical building mechanical roomor basement and allow for the existing drains lines to enter and exitthe device. The device would use gravity and water main pressurewhenever possible to conserve energy. The device should be easilyinstalled in current plumbing and require minimal maintenance. Thedevice should also be able raise the temperature of the incoming coldwater by 30° F. to remain competitive with devices currently on themarket. Currently there is no single device on the market which willtake in grey water from large numbers of showers and capture the wastedenergy. The design of a system which could efficiently capture thiswasted energy and use it to pre-heat the incoming cold water coulddrastically lower the energy usage in these commercial areas.

The design that was chosen is a helical pipe in a pipe design. Thisdesign improves upon the shortcomings of other heat exchanger designs.The design calls for the hot wastewater to flow over the helical copperpipe which contains the incoming cold water that is being sent to theshowers. This design allows for gravity to drive the hot wastewaterthrough the system and out of the heat exchanger.

The helical pipe design maximizes the area of contact because it isplaced directly in the hot grey water flow. This set-up captures thelargest amount of energy from the water. Due to the nature of thehelical pipe, debris build up can be prevented by using an appropriatespacing between the coils. The outer pipe size was determined byequating the unmodified drainage pipe size to a larger pipe with thehelical pipe inside. This allows for the same flow rates that a typicalunmodified drain pipe can accommodate as well as preventing additionalclogging.

The design specifications were taken into consideration when determiningthe necessary pipe dimensions, spacing and length for the heatexchanger. The length of the copper tubing determines how much heattransfer will take place. The longer the pipe, the more heat will betransferred until an equilibrium is reached. The design has been reducedto practice and has been tested successfully.

Pipe in a Pipe Design

Depicted below in FIG. 1A and FIG. 1B is the pipe in a pipe heatexchanger, the most basic heat exchanger possible, which was analyzedfirst. D_(i) represents the inner diameter of the inside copper pipewhere the incoming cold water would flow and D_(o) represents the innerdiameter of the outside PVC pipe where the hot grey water would passover the cold water pipe.

The conditions used in the analysis were ideal to achieve a best casescenario. Assumptions included an adiabatic system, negligible changesin potential and kinetic energy, and constant fluid properties. It alsodid not consider the thermal resistance of the pipes or fouling factors.The flow of the cold water was found to be turbulent in the inner pipe,while the hot water flow in the annulus was found to be laminar. Aftercompleting this analysis for a group of ten showers, the length of pipeneeded to raise the domestic cold water 5° C. with incoming cold waterat 12° C. and grey warm water leaving at 27° C. is 120.29 meters. Thesevalues were obtained using methods described later in section 6.2. Usingthe length found in the ideal case, this number was determined to be theminimum pipe length needed. This case would not be a practical designand cannot be implemented at reasonable cost, but it can be used as abasis for comparison and insight into other designs.

Shell and Tube Design

The shell and tube counter flow heat exchanger design was the nextdesign under consideration. This design would consist of conductingnumerous passes of the grey water over the cold water pipes. It wasdetermined that the shell and tube design would not meet the designspecifications due to the amount of space needed to pass the grey waterpipes by the cold water pipes. The design would require a similar lengthof pipe as was found for the case of the pipe in a pipe design, weavedinto a closed space.

Helical Coil in a Pipe Design

The current design was conceived to eliminate the disadvantages of theprevious designs. This design includes increasing the contact area ofthe copper coil to the hot grey water flow in a shorter length ofvertical pipe, therefore increasing contact time between the two flowsand allowing for the capture of more recoverable energy. The capture ofthe greatest amount of energy possible is also aided by increasing theturbulent flow within the pipes. By placing a helical coil in the middleof the pipe and providing an appropriate flow rate into the heatexchanger, the turbulent flow can be increased. The advantages of theprevious two designs are also still present in this design; theseadvantages include the ease of installation and maintenance.

The helical copper coil inserted into a vertical PVC pipe providesseveral advantages. The design exhibits cross flow conditions which areoptimal conditions for liquid heat exchangers. The helical copper coilcontains the cold (clean) incoming water and the outer PVC pipe containsthe hot grey water. The cold incoming water is fed in through the bottomof the heat exchanger and is warmed by the hot grey water flowing downover it. The heated clean water exits via a copper pipe near the top ofthe PVC pipe. The grey water exits through the bottom of the PVC pipe.This will be enclosed with additional insulation around the outside ofthe outer PVC pipe, providing for adiabatic conditions around theoutside of the heat exchanger. FIG. 2 is a perspective view of a helicalcoil in a pipe heat exchanger without insulation.

In FIG. 3, the hot grey water is found in the white areas which aredefined by D_(o) which is the diameter of the outer PVC pipe and x whichis the inner diameter of the copper coil. The variable d is the diameterof the helical coil containing the cold incoming water. These hot andcold flows should never come into contact with one another, to preventthe contamination of the incoming clean cold water.

FIG. 4 is a vertical cross-section of the design, the same variableshold true. The warm grey water is denoted by the red arrows and seengoing through the center dimension as well as down the sides and finallyout into of the heat exchanger. The cold clean incoming water is denotedby the blue arrows going up through the helical pipe.

The major challenge with the helical in pipe design was the potentialfor clogging. This potential for clogging is diminished by increasingthe spacing between coils in the helical pipe. This spacing is 0.125″and can be seen in FIG. 5. The spacing allows for the grey water to passfrom one side oldie coil to the other. By allowing this passage, itflushes out any possible debris that could potentially clog andtherefore reduce the efficiency of the heat exchanger.

The location and intended use of the heat exchanger can be seen in FIG.6. The heat exchanger will pre-heat cold incoming water destined for theshowers. This water combines in a mixing valve at the shower whichregulates the temperature of the water coming out of the shower head.This allows for the boiler to heat less water and use less energybecause the desired temperature can be obtained with less hot water. Thedesign also provides for the use of gravity, as shown in FIG. 6, to movethe flow of the hot grey water down the heat exchanger and ultimately tothe sewer. The water main pressure allows for the cold clean incomingwater to make it to the top of the heat exchanger with a slight pressuredrop of 0.618 psi, determined through calculations. This pressure dropshows that the design does not need a pump to assist in moving the waterthrough the heat exchanger and on its way through the rest of theplumbing system.

Helical Coil in a Pipe Analysis

To begin analysis on the chosen full scale design, the flow conditionsin the heat exchanger needed to be determined. These conditions includethe flow rates of both the cold water and the hot grey water as well asthe temperatures each of these would be entering the heat exchanger.Flow rates were determined from the widespread use of low-flow showerheads mandated across the United States. These low-flow showerheadsallow 2.5 gallons per minute (gpm) of water to exit, down from 5 gpmthat older showerheads allowed. Since the hot shower water coming out ofthe showerhead is the same water going down the drain towards the heatexchanger, this 2.5 gpm became the flow rate of warm grey water pershower for all analyses. Shower water is a mixture of cold water comingfrom a well or town supply and hot water heated in a boiler.Approximately 0.5 gpm of the 2.5 gpm shower Water flow comes from thecold water pipeline. The 0.5 gpm of cold water being mixed is the samethat flows through the heat exchanger prior to entering the showermixing valve and is therefore the flow rate used in analysis. The heatexchanger was analyzed for ten showers happening at the same time, soflow rates of 25 gpm grey water and 5 gpm cold water were used. Throughresearch and testing it was determined that the average temperature ofthe cold water entering from the street, T_(ci), and thus the heatexchanger, is 48° F. and the temperature of grey shower water going downa drain, T_(hi), is 104° F. Projected exit temperatures were used todetermine an average temperature for each of the water types, which werethen used to find the specific heat, thermal conductivity, viscosity,and Prandtl Number for each flow.

In order to calculate the dimensions of the full scale heat exchanger, aseries of optimizations were performed. The first sets of dimensions tobe analyzed were the cross-sectional dimensions. These include thediameter of the outer PVC pipe, D_(o), the diameter and thickness of thecopper coiled pipe, d and t respectively, and the diameter of the coilitself, X. Values for D_(o) and t were chosen from a list of availablesizes from McMaster-Carr. The wall thickness of the copper coil dependsdirectly on the diameter of the pipe and was not treated as anindependent variable. The idea behind the optimization was to providethe maximum heat transfer while minimizing costs and maintaining designintegrity. One design feature that was addressed in this optimizationwas making sure the heat exchanger did not impede the normal flow ofdrain water. This was accomplished by equating the area of a standardfour inch diameter drain pipe to the area the grey water was allowed toflow through in the chosen design. This relationship is seen below inEquation 1. Since Do and t had a finite set of values, the equation wassolved for X, the modified equation is shown in Equation 2. From thisequation an optimization table was populated that yielded maximum valuesof X for certain combinations of Do and d. The inequality expressed inEquation 3 was then used to determine which values of X were feasiblevalues that did not have the copper coil existing outside of the PVCpipe. The nine values of X that satisfied the inequality became designdimension considerations. The optimization table can be seen below inTable 1, with the design consideration values italicized in red text.These design considerations were then analyzed in terms of the heattransfer they produced in a six foot tall heat exchanger as well as thecost to produce a one foot length of a heat exchanger with the givendimensions. The method used in determining both of those values will bediscussed below.

$\begin{matrix}{{\frac{1}{4}{\pi (4)}^{2}} = {\frac{1}{4}{\pi \left( {D_{0}^{2} - \left( {X + {2d} + {4t}} \right)^{2} + X^{2}} \right)}}} & (1) \\{X = \frac{D_{0}^{2} - \left( {{2d} + {4t}} \right)^{2} - 16}{2\left( {{2d} + {4t}} \right)}} & (2)\end{matrix}$X+2d+4t<D ₀ ²  (3)

TABLE I Cross Section Dimensional Optimization Do d t 5.761 6.065 7.6257.981 0.527 0.049 6.250 7.688 16.231 18.453 0.666 0.042 4.979 6.17813.296 15.148 0.995 0.065 2.694 3.497 8.239 9.474 1.245 0.065 1.7502.403 6.286 7.297 1.481 0.072 1.019 1.572 4.858 5.712 2.009 0.058 −0.1020.320 2.832 3.486

Cross flow heat exchangers transfer heat between liquids that areflowing roughly perpendicular to each other. The coiled copper pipeinside of the PVC outer pipe closely resembles cross flow conditions,where the cold water is considered unmixed and the warm grey water isconsidered mixed. As such, the chosen design will be analyzed as a crossflow heat exchanger.

To begin cross flow heat exchanger analysis the heat capacity, C, ofeach liquid has to be determined. This is found by multiplying the massflow rate of the liquid, m, with its specific heat, c_(p), as shown inEquation 4. The heat capacity is calculated for both the hot grey waterand cold water flows and the flow with the smaller heat capacity isdenoted as C_(min). For this design, the cold water flow was found to beC_(min) and the hot grey water flow was found to be C_(max). The valueC_(r) is the ratio of C_(min) to C_(max) and can be seen in Equation 5.

C={dot over (m)}×c _(p)  (4)

$\begin{matrix}{C_{r} = \frac{C_{\min}}{C_{\max}}} & (5)\end{matrix}$

The next step in the analysis is to calculate the Reynolds' Numbers forboth the cold water flow and the hot grey water flow. The cold waterReynolds' Number is found through Equation 6 where μ_(c) is theviscosity of the cold water. The hot grey water flow depends on thehydraulic diameter, D_(h), wetted perimeter, P_(w), and contact area, A,of the design. The relationships between the three variables are shownbelow in Equations 7, 8 and 9. The Reynolds' Number of the hot greywater flow is calculated in Equation 10.

$\begin{matrix}{{Re}_{C} = \frac{4{\overset{.}{m}}_{C}}{\mu_{C}d\; \pi}} & (6) \\{D_{h} = \frac{4A}{P_{W}}} & (7) \\{A = {\frac{1}{4}{\pi \left( {D_{0}^{2} - \left( {X + {2d} + {4t}} \right)^{2} + X^{2}} \right)}}} & (8)\end{matrix}$P _(W)=π(D ₀+(X+2d+4t)+X  (9)

$\begin{matrix}{{Re}_{h} = \frac{4{\overset{.}{m}}_{h}}{\mu_{h}P_{W}}} & (10)\end{matrix}$

Both flows exhibit turbulent flow, as can be seen by the Reynolds'Numbers of 14,715 and 8,515 as well as the complexity of the design. TheNusselt's Number for both turbulent flows is calculated with Equation11, and is then used to calculate the convection coefficients, h_(c) andh_(h). The cold water convection coefficient is found using Equation 12and the hot grey water convection coefficient is found using Equation13.

Nu=0.023Re^(0.8)Pr^(0.4)  (11)

$\begin{matrix}{h_{C} = {{Nu}_{C}\frac{k_{fC}}{d}}} & (12) \\{h_{h} = {{Nu}_{h}\frac{k_{fh}}{D_{h}}}} & (13)\end{matrix}$

The stretched out length of the copper coil is calculated using Equation14 where H is the desired height of the heat exchanger and s_(c) is thespacing between consecutive coils. This length is used to determine thesurface area of the copper coil, which is also the heat transfer areaused in the heat exchanger. The surface area is broken up into twocalculations; cold side area, A_(c), and hot side area, A_(h). Thedifference between the two areas is that the hot side area takes thecopper pipe's thickness into consideration. Calculations for both areasare shown in Equations 15 and 16. Using the areas and convectioncoefficients calculated above, along with the copper pipe thickness andthermal conductivity, k; the overall convection coefficient across thearea of the heat exchanger, UA_(c), was calculated. A graphicalrepresentation of the heat flux is shown in FIG. 7 and the equationderived from that is shown in Equation 17.

$\begin{matrix}{L = \frac{{\pi \left( {X + d} \right)}\left( {H - d} \right)}{\left( {d + s_{c}} \right)}} & (14)\end{matrix}$A_(C)=πdL  (15)

A _(h)=π(d+2t)L  (16)

$\begin{matrix}{{UA}_{C} = \frac{1}{\frac{1}{h_{C}A_{C}} + \frac{t}{k\; A_{C}} + \frac{1}{h_{h}A_{h}}}} & (17)\end{matrix}$

Once the overall convection coefficient is determined, the number oftransfer units, NTU, and the efficiency of the heat exchanger, ε, can becalculated. The maximum heat transfer, q_(max), and the actual heattransfer, q, can also be calculated. The equations that relate all ofthese values are shown below in Equations 18, 19, 20 and 21.

$\begin{matrix}{{NTU} = \frac{{UA}_{C}}{C_{\min}}} & (18) \\{ɛ = {\left( \frac{1}{C_{r}} \right)\left( {1 - ^{({- {C_{r}{({1 - ^{- {NTU}}})}}})}} \right)}} & (19)\end{matrix}$q _(MAX) =C _(min)(T _(hi) −T _(ci))  (20)

q=ε·q _(MAX)  (21)

Using the actual heat transferred one can find the outlet hot and coldtemperatures, T_(ho) and T_(co) respectively. The formulas to do so areshown in Equations 22 and 23.

$\begin{matrix}{T_{ho} = {T_{hi} - \frac{q}{{\overset{.}{m}}_{h}{cp}_{h}}}} & (22) \\{T_{Co} = {T_{Ci} + \frac{q}{{\overset{.}{m}}_{c}{cp}_{c}}}} & (23)\end{matrix}$

Now that the dimensions of the heat exchanger can be related to atemperature increase in the cold water, the prototype cost per foot willalso be related to the cross-sectional dimensions. This will allow forthe optimization started above to narrow down the possible designdimension considerations. To obtain a cost per foot for each of thepotential design dimensions, prices for each of the PVC and copper pipesizes were obtained. One foot of heat exchanger includes one foot of PVCpipe and the length, L, of copper pipe that can be coiled inside. Thislength was found earlier in Equation 14. Table 2 was then constructed,displaying the dimensions of each consideration along with the pricingper foot to prototype. Designs 2, 3, 6, and 7 were not considered forcost analysis since the maximum temperature they could put out was farless than the other designs.

Designs 5, 8, and 9 were then removed from consideration due to costingmore and producing lower temperature results than the other designs.Designs 1 and 4 were then modified to nominal dimensions of X andacceptable values for the spacing between the outside of the coil andthe inside PVC wall. The final design chosen was the modified version ofDesign 1 which has a PVC diameter of 5.709″, a copper coil diameter of0.995″, and a value of 2.7″ for X. This design yielded a change intemperature of the cold water equal to 31.4° F. over six feet of heatexchanger.

TABLE 2 Heat Transfer and Prototype Cost Optimizations L_(H=1 ft) ΔTDesign D₀ (in) d (in) t (in) X (in) (in) (° F.) $/Foot 1 5.761 0.9950.065 2.695 104.402 30.957 $30.96 2 5.761 1.45 0.065 1.750 69.547 24.991— 3 5.761 1.481 0.072 1.019 49.754 20.080 — 4 6.065 0.995 0.065 3.492126.238 34.563 $34.56 5 6.065 1.245 0.065 2.404 84.093 28.490 $28.49 66.065 1.481 0.072 1.573 59.555 23.233 — 7 6.065 2.009 0.058 0.320 33.71515.196 — 8 7.625 2.009 0.058 2.833 68.358 26.280 $26.28 9 7.981 2.0090.058 3.486 77.370 28.528 $28.53

The most common way to heat water in commercial applications is with anatural gas boiler. As such, all cost savings analysis will be performedassuming a natural gas boiler is being used. Cost savings weredetermined for an individual shower operating under the conditionsstated in the design specifications, primarily that ten showers arerunning at once. This was accomplished by performing the analysis at aten shower flow load and then dividing by ten to get an individual heattransfer value, in Joules per second. To transfer this into energy, theheat transfer was multiplied by the length of an average shower, tenminutes. This energy, Q, is the energy that the boiler or water heaterno longer needs to provide to the water for shower use, and is thereforesaved. To convert this to cost savings, the energy saved is divided bythe boiler efficiency, 0.8 in the case of natural gas boilers, thendivided by the energy produced per cubic foot of natural gas, thenmultiplied by the price of natural gas per cubic foot. The equation thatrepresents this conversion is shown in Equation 24. To change this intoa yearly savings, a scenario of a gym that services 200 showers a day,under ten showers at a time conditions, is assumed.

$\begin{matrix}{{\frac{Q}{0.8}*\left( \frac{1\mspace{14mu} {ft}^{3}}{1.80776*10^{6}\mspace{14mu} J} \right)*\left( \frac{{\$ 15}{.43}}{1000\mspace{14mu} {ft}^{3}} \right)} = {\$/{Shower}}} & (24)\end{matrix}$

The next dimension of the heat exchanger to optimize was the overalllength. A graph of the outlet temperatures of the chosen heat exchangerdimension vs. the length was created and shown in FIG. 8. The amount ofheat transfer gained by adding another foot of length gets smaller asthe heat exchanger gets larger. Since the price of the heat exchangerper foot remains constant, and the cost savings associated with the heatexchanger depend on how much heat is transferred, an optimal length canbe determined. FIG. 9 shows the cost savings per year as well as thecost to prototype as a function of length. FIG. 10 shows the differencebetween the cost savings and the prototype cost. As seen in FIG. 10, theoptimal length of a heat exchanger with the chosen dimensions is betweensix and eight feet long. A length of six feet was selected for the finaldesign to fit within the design specifications. At this length, theexpected cost savings for a 10 shower gym are $600 for the first yearand $1050 a year for subsequent years.

The complete analysis of the chosen design and optimized dimensions canbe seen in Table 3.

Test Set-Up

In order to validate the theoretical calculations a prototype device wascreated. A scaled down model of the final design was chosen as theprototype because of manufacturability and limitations of the testingconditions available such as water flow rates. The scaled down modelreduced the PVC pipe diameter to 4″, the copper pipe diameter to 0.31″,the diameter of the coil to 2″ and the vertical height of the coppercoil to 3′. Due to manufacturability, the spacing between the coils wasincreased to 0.3″. The flow rate available in the testing lab was amaximum of 12.5 gpm, with maximum hot water temperature of 120° F. and acold water temperature of 53° F. It was decided to assume a 5 showerload which would require a hot water flow of 12.5 gpm and a cold waterflow of 2.5 gpm. The dimensions of the scaled model were placed into thecalculations spreadsheet and the expected outlet temperature of the coldwater was found to be 67.82° F., an increase in temperature of 14.49° F.The analysis for the scaled model can be seen in Table 4. Test resultsthat prove that the scaled down model works would also confirm the fullscale model's behavior.

Temperatures during the test were measured through the use of 4 K-typethermocouples. The thermocouples were placed inside the pipes at theinlets and outlets of both the hot water and the cold water. Epoxy wasused to keep the thermocouples in place and sealant was used to preventleaks at connection points on the device. A LabVIEW VI was created toacquire and record data from the tests.

Water was brought to the heat exchanger by 0.75″ garden hoses andseveral connections that attached to either the copper tubing or the PVCpipe. Ball valves were included on all water lines to control flow andas an added safety measure. Once the set-up was fully attached, flowrates of the water through the heat exchanger were obtained using a 5gallon bucket and a stopwatch. The set-up of the test apparatus can beseen in FIG. 11.

Six tests were conducted to verify the operation of the heat exchanger.The design was tested under two different flow conditions. During thefirst configuration the PVC pipe was half filled with water due to theuse of a restriction on the hot water exiting the heat exchanger. Thesecond configuration tested was without a restriction on the hot waterflow outlet which did not allow for the hot water to back up inside thePVC pipe.

The tests performed with the restriction can be seen in action in FIG.12. This figure demonstrates how the hot water half-filled the PVC pipewhich created a very turbulent flow in that section. FIG. 13, FIG. 14and FIG. 15 show the temperature plots of the testing when therestriction was applied. As can be seen in FIG. 13 through FIG. 15, thechange in temperature and cold water outlet temperature follow thefluctuations of the incoming hot water.

The tests performed without the restriction can be seen in action inFIG. 16. This figure shows the water going straight through the PVC pipewithout any hot water backing up into the pipe. FIG. 17, FIG. 18 andFIG. 19 show the temperature plots for the testing without therestriction in place. As can be seen in FIG. 17, FIG. 18 and FIG. 19,the change in temperature and cold water outlet temperature follow thefluctuations of the incoming hot water, which also occurred when therestriction was in place. This configuration would most accuratelyreplicate conditions seen in the full scale device.

The testing results show that the heal exchanger performed slightlybetter with no restriction on the drain pipe. The change in temperatureof the cold water was 38.5° F. for the restricted flow, whichcorresponds to an outlet temperature of 90.5° F. The change in coldwater temperature for the unrestricted flow was 42° F. which correspondsto an outlet temperature of 95° F.

Data acquired during testing shows the scaled prototype performing muchbetter than expected. The reasoning behind this is the hot water flow ismuch more complex than modeled in the calculations spreadsheet. Theeffect of placing the copper coil in the middle of the grey water flowand the use of spacing between the coils were not accounted for in theinitial calculations of the Nusselt's numbers. The complexity of thedesign prevents the heat transfer behavior from being fully understoodin theory alone, experimentation is needed. From the tests it wasdetermined that the theoretical Nusselt's number should be increased toproperly model the actual performance of the heat exchanger. When thischange is applied to the full scale design, the outlet temperature ofthe cold water increases from 79.4° F. to 93.8° F. this updated analysiscan be seen in able 5. The outlet temperature corresponds to an annualsavings of $1400 for a typical gym with 10 showers and 200 uses per day.

The difference in results between the two configurations can beattributed to the column of water that builds up in the PVC pipe whenthe flow is restricted. The column of water immerses the bottom halfoldie copper coil completely which increases contact area, but it comesat a loss of hot water temperature. The differences between the inletand outlet temperatures of the grey water for each test are shown inTable 4. The average change in the grey water temperature forunrestricted flow was 3.44° F. compared to 3.72° F. for restricted flow.This difference is translated to the cold water flow, which is verysensitive to the changing hot water temperature.

Table 6 presents results of analysis for a full scale unit.

Filter

A heat exchanger is a device used to transfer heat from one medium toanother, in our case, the energy from the grey water to the incomingcold water. The design's geometry was conceptualized to efficientlytransfer this heat energy, and at the same time, not drastically affectthe discharge of waste water.

The design was to put a helical pipe inside of a larger drainage pipe.The design calls for the grey water to flow over the inner helical pipewhich contains the incoming cold water that is being sent to theshowers. This allows for gravity to drive the hot wastewater through thesystem and out of the heat exchanger. This can be seen in FIG. 20.

The helical pipe design maximizes the area of contact as it is placeddirectly in the flow of the grey water, capturing the largest amount orenergy from the water. Due to the nature of the helical pipe, debrisbuild up can be prevented by using an appropriate spacing between thecoils and by including a filtering system described later. The outerpipe size was determined by equating the unmodified drainage pipe sizeto a larger pipe with the helical pipe inside. This allows for the sameflow rates that a typical unmodified drain pipe can accommodate as wellas preventing additional clogging.

The next feature of the grey water heat recovery system providesprotection to the systems helical coil by filtering the grey water. Thefilter prevents debris buildup which not only leads to clogging but alsoinhibits the transfer of energy. The filter is modular and is insertedinto the drainage pipe above the helical coil using a unique system ofclasps and seals to ensure water tight fit.

Another development to prevent debris build up in the heat exchangerconsists of a filtering device. This was done by creating a slot for thefilter module in the large drainage pipe. The filter module is a simpleyet effective design of a cup with a filter on the bottom. The filterprevents debris buildup which ensures the efficiency of the machine isnot affected. When time to replace, the customer would realize a slowerdraining rate, signaling the need to replace the filter, all whilekeeping the coil protected. The filter will be placed in a circularrecess that has the same inner radius and thickness as the largedrainage pipe. This creates a tight fit allowing for complete debriscollection. The makeup of the outer portion oldie filter will consist ofa half-pipe with the same radius as the large pipe. The filter will besecured with two latches at either end of the half-pipe that will hookinto the heat exchangers drainage pipe, locking the filter in place fora water tight fit. The filter module will include an indicator mechanismto confirm to the user, after slower draining is experienced, that thefilter needs to be cleaned. The filter module without the actual filtercan be seen in FIG. 21.

There are several advantages of incorporating a filter into the systemdesign. The filter will reduce the speed of the incoming grey water inaddition to creating a more turbulent flow and filtering the water in acentralized location. Slowing the velocity of the water allows forlonger exposure to the coils, thereby increasing the heat transfer. Theonly downside to the modular filter design is the added maintenance tothe system. The frequency of maintenance will depend on the system useand the amount of debris from the environment the system is used in.

The design with the use of the filter has the potential to back up withdebris once the filter is filled. This is solved by using a pipeconnection called a tee, which has two outlets, similar to the shape ofthe letter “T”. By placing the tee with the single end going into theheat exchanger system, the second branch of the tee connector allows forthe start of the bypass, which runs down the side of the pipe. Plumbingthe system this way allows for a place where the backed up water can goif the filter is clogged, creating a bypass of the heat exchanger. Fromthe tee, the bypass pipe is plumbed to down the reverse side of thesystem. This pipe provides rigidity where the filter insert getsinserted. The bypass pipe runs parallel to the heat exchanger until itreunites with the sewage line. This can be seen in FIG. 20.

FIG. 22 is an exploded diagram of one embodiment of a helical coilcounterflow heat exchanger that illustrates the parts of the invention.A tee pipe fitting 1 admits waste water at the top and connects to boththe heat exchanger and the bypass pipe. The path leading to the heatexchanger then has a pipe increaser 2 which then leads to the filtermodule 3. The grey water proceeds down pipe 7 and flows over theexterior of a helical coil 8. The helical coil 8 has a connector 5 ateach end that is provided for ease of manufacturing and to position thehelical coil 8 in the center of the drainage pipe 7. Clean cold waterflows upwardly in the helical coil 8 when recovery of heat from the greywater is taking place. Once the grey water has passed over the helicalcoil 8 it proceeds to an elbow 9, followed by a short length of pipe 10of the same diameter. Finally the water passes through a pipe reducer 12which leads to the rest of the water discharge plumbing. The bypassroute starts at the teel, to an elbow4, followed by a straight pipe 6.The bypass is completed with an elbow 4 and then can be connected to therest of the water discharge plumbing.

Flow Control and Thermal Management

The flow system includes a switch and a controller to control a valve todirect the flow of grey water through the heat exchanger. In someembodiments, the controller is a thermal switch. In some embodiments,the thermal switch is a bimetallic element that operates to allow greywater to flow through the discharge pipe when the grey water has atemperature above a predetermined value (e.g., is warm enough to provideuseful thermal energy), and to cause the grey water to flow thorough abypass pipe when the grey water has a temperature below a predeterminedvalue (e.g., is not warm enough to provide useful thermal energy). Insome embodiments, the controller is an electronic microprocessor-basedcontroller configured to operate under instructions provided on amachine-readable medium. In some embodiments, the controller furthercomprises a thermal sensing element, such as a thermocouple, that canprovide a non-volatile electrical signal representative of a temperature(e.g., the temperature of the grey water). In some embodiments, thecontroller further comprises a flow sensing element that can sensewhether grey water is flowing in the discharge pipe or in the bypasspipe. When no grey water is flowing at all, there is no point in runningclean water through the heat exchanger. For example, the controllercontrols the bypass valve to cause grey water to flow through the bypasspipe when the filter becomes clogged. The controller can indicate such acondition by announcing that the filter is clogged and should be cleanedor replaced.

DEFINITIONS

Recording the results from an operation or data acquisition, such as forexample, recording results at a particular frequency or wavelength, isunderstood to mean and is defined herein as writing output data in anon-transitory manner to a storage element, to a machine-readablestorage medium, or to a storage device. Non-transitory machine-readablestorage media that can be used in the invention include electronic,magnetic and/or optical storage media, such as magnetic floppy disks andhard disks; a DVD drive, a CD drive that in some embodiments can employDVD disks, any of CD-ROM disks (i.e., read-only optical storage disks),CD-R disks (i.e., write-once, read-many optical storage disks), andCD-RW disks (i.e., rewriteable optical storage disks); and electronicstorage media, such as RAM, ROM, EPROM, Compact Flash cards, PCMCIAcards, or alternatively SD or SDIO memory; and the electronic components(e.g., floppy disk drive, DVD drive, CD/CD-R/CD-RW drive, or CompactFlash/PCMCIA/SD adapter) that accommodate and read from and/or write tothe storage media. Unless otherwise explicitly recited, any referenceherein to “record” or “recording” is understood to refer to anon-transitory record or a non-transitory recording.

As is known to those of skill in the machine-readable storage mediaarts, new media and formats for data storage are continually beingdevised, and any convenient, commercially available storage medium andcorresponding read/write device that may become available in the futureis likely to be appropriate for use, especially if it provides any of agreater storage capacity, a higher access speed, a smaller size, and alower cost per bit of stored information. Well known oldermachine-readable media are also available for use under certainconditions, such as punched paper tape or cards, magnetic recording ontape or wire, optical or magnetic reading of printed characters (e.g.,OCR and magnetically encoded symbols) and machine-readable symbols suchas one and two dimensional bar codes. Recording image data for later use(e.g., writing an image to memory or to digital memory) can be performedto enable the use of the recorded information as output, as data fordisplay to a user, or as data to be made available for later use. Suchdigital memory elements or chips can be standalone memory devices, orcan be incorporated within a device of interest. “Writing output data”or “writing an image to memory” is defined herein as including writingtransformed data to registers within a microcomputer.

“Microcomputer” is defined herein as synonymous with microprocessor,microcontroller, and digital signal processor (“DSP”). It is understoodthat memory used by the microcomputer, including for exampleinstructions for data processing coded as “firmware” can reside inmemory physically inside of a microcomputer chip or in memory externalto the microcomputer or in a combination of internal and externalmemory. Similarly, analog signals can be digitized by a standaloneanalog to digital converter (“ADC”) or one or more ADCs or multiplexedADC channels can reside within a microcomputer package. It is alsounderstood that field programmable array (“FPGA”) chips or applicationspecific integrated circuits (“ASIC”) chips can perform microcomputerfunctions, either in hardware logic, software emulation of amicrocomputer, or by a combination of the two. Apparatus having any ofthe inventive features described herein can operate entirely on onemicrocomputer or can include more than one microcomputer.

General purpose programmable computers useful for controllinginstrumentation, recording signals and analyzing signals or dataaccording to the present description can be any of a personal computer(PC), a microprocessor based computer, a portable computer, or othertype of processing device. The general purpose programmable computertypically comprises a central processing unit, a storage or memory unitthat can record and read information and programs using machine-readablestorage media, a communication terminal such as a wired communicationdevice or a wireless communication device, an output device such as adisplay terminal, and an input device such as a keyboard. The displayterminal can be a touch screen display, in which case it can function asboth a display device and an input device. Different and/or additionalinput devices can be present such as a pointing device, such as a mouseor a joystick, and different or additional output devices can be presentsuch as an enunciator, for example a speaker, a second display, or aprinter. The computer can run any one of a variety of operating systems,such as for example, any one of several versions of Windows, or ofMacOS, or of UNIX, or of Linux. Computational results obtained in theoperation of the general purpose computer can be stored for later use,and/or can be displayed to a user. At the very least, eachmicroprocessor-based general purpose computer has registers that storethe results of each computational step within the microprocessor, whichresults are then commonly stored in cache memory for later use.

Many functions of electrical and electronic apparatus can be implementedin hardware (for example, hard-wired logic), in software (for example,logic encoded in a program operating on a general purpose processor),and in firmware (for example, logic encoded in a non-volatile memorythat is invoked for operation on a processor as required). The presentinvention contemplates the substitution of one implementation ofhardware, firmware and software for another implementation of theequivalent functionality using a different one of hardware, firmware andsoftware. To the extent that an implementation can be representedmathematically by a transfer function, that is, a specified response isgenerated at an output terminal for a specific excitation applied to aninput terminal of a “black box” exhibiting the transfer function, anyimplementation of the transfer function, including any combination ofhardware, firmware and software implementations of portions or segmentsof the transfer function, is contemplated herein, so long as at leastsome of the implementation is performed in hardware.

Theoretical Discussion

Although the theoretical description given herein is thought to becorrect, the operation of the devices described and claimed herein doesnot depend upon the accuracy or validity of the theoretical description.That is, later theoretical developments that may explain the observedresults on a basis different from the theory presented herein will notdetract from the inventions described herein.

Any patent, patent application, or publication identified in thespecification is hereby incorporated by reference herein in itsentirety. Any material, or portion thereof, that is said to beincorporated by reference herein, but which conflicts with existingdefinitions, statements, or other disclosure material explicitly setforth herein is only incorporated to the extent that no conflict arisesbetween that incorporated material and the present disclosure material.In the event of a conflict, the conflict is to be resolved in favor ofthe present disclosure as the preferred disclosure.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be affected therein without departing from the spirit andscope of the invention as defined by the claims.

1. A grey water to clean water heat exchanger, comprising: a dischargepipe having an exterior surface, an interior diameter, a grey waterinlet and a grey water outlet, said discharge pipe configured to conveygrey water from a location of use to a discharge location; a helicalcoil having a clean water inlet and a clean water outlet, said helicalcoil situated within said interior diameter of said discharge pipe withsaid clean water inlet and said clean water outlet configured to beaccessible from locations outside said exterior surface of saiddischarge pipe, said helical coil configured to carry clean water from awater source to a location of use of the clean water, said dischargepipe and said helical coil configured to allow heat energy to flow fromsaid grey water to said clean water; a filter configured to be removablypositioned within said grey water flow, said filter configured to filtermaterial from said grey water; a bypass valve and a bypass pipeconfigured to allow grey water to controllably bypass said dischargepipe and said helical coil and to reach said discharge location; and acontroller configured to activate and deactivate said bypass valve inresponse to a non-volatile control signal.
 2. The grey water to cleanwater heat exchanger of claim 1, wherein said discharge pipe and saidhelical coil are configured to have said grey water and said clean waterflowing in counterflow to each other.
 3. The grey water to clean waterheat exchanger of claim 1, wherein said controller is a thermal switch.4. The grey water to clean water heat exchanger of claim 3, wherein saidthermal switch is a bimetallic element.
 5. The grey water to clean waterheat exchanger of claim 1, wherein said controller is an electronicmicroprocessor-based controller configured to operate under instructionsprovided on a machine-readable medium.
 6. The grey water to clean waterheat exchanger of claim 5, wherein said controller further comprises athermal sensing element.
 7. The grey water to clean water heat exchangerof claim 6, wherein said thermal sensing element is a thermocouple. 8.The grey water to clean water heat exchanger of claim 1, wherein saidcontroller further comprises a flow sensing element.
 9. The grey waterto clean water heat exchanger of claim 1, wherein said helical coilcomprises copper.
 10. The grey water to clean water heat exchanger ofclaim 1, wherein said filter comprises a nylon filter element.
 11. Athermal energy recovery method, comprising the steps of: providing agrey water to clean water heat exchanger, comprising: a grey waterdischarge pipe configured to carry grey water and a helical coilsituated within said discharge pipe, said helical coil configured tocarry clean water, a removable filter configured to filter material fromsaid grey water, a bypass valve and a bypass pipe configured to allowgrey water to controllably bypass said discharge pipe, and a controllerconfigured to activate and deactivate said bypass valve in response to anon-volatile control signal; providing a source of grey water and asource of clean water; setting said bypass valve to convey grey waterthrough said bypass pipe; sensing whether a flow of grey water having atemperature above a predetermined value is present in said bypass pipe;in response to sensing that a flow of grey water having a temperatureabove a predetermined value is present in said bypass pipe, causing saidcontroller to operate said bypass valve to flow said grey water throughsaid discharge pipe; and recovering thermal energy from said grey waterand heating clean water in said helical coil with said recovered energy.12. The thermal energy recovery method of claim 11, wherein saidcontroller controls said bypass valve to cause grey water to flowthrough said bypass pipe when said filter becomes clogged.
 13. Thethermal energy recovery method of claim 11, wherein said controllerindicates that said filter requires cleaning or replacement when saidfilter becomes clogged.